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Abstract

Disclosed is an overlay measuring method which uses two measurements with different focus to significantly reduce the measuring error.

Country

United States

Language

English (United States)

This text was extracted from an ASCII text file.

This is the abbreviated version, containing approximately
68% of the total text.

Overlay Measurement with Increased Accuracy

Disclosed is
an overlay measuring method which uses two
measurements with different focus to significantly reduce the
measuring error.

Today's
overlay measuring tools for the 350 nm linewidth chip
generation have a measuring accuracy of about 7.5 nm. An analysis of
the overlay measuring accuracy reveals that the displacement induced
by the measuring tool is about 5 nm.

The measuring
tools mostly used are TV microscopes with white
light illumination and with objectives having a high numerical
aperture N.A.. The depth of focus of
these objectives is
T=lambda./N.A.. With typical values of
lambda.=0.5 microsecond m and
N.A.=0.95, T is about 0.5 microsecondm.
The height differences of
the overlay marks may, however, be up to about 1 microsecond m.

The overlay
marks lay within the depth of focus of the
objective but significantly away from the ideal focus. These small
distances lead to a displacement induced by the measuring tool if the
illumination is oblique or not symmetrical.

There are two
possibilities to avoid the tool induced
displacement: using symmetrical illumination or eliminating the
height difference of the overlay marks.

The
illumination is almost symmetrical in today's measuring
tools. The solution is shown in the
drawing. A real image is
projected onto one TV camera and a virtual image is projected onto
another TV camera via a beam splitter. ...